In practice though, getting the AST from the text is a computational task in and of itself and the grammar affects the runtime of that. For instance, Rust's "tu

In practice though, getting the AST from the text is a computational task in and of itself and the grammar affects the runtime of that. For instance, Rust's "turbofish" syntax, `f::<T>(args)`, is used to specify the generic type for f when calling it; this is instead of the perhaps more obvious `f<T>(args)`, which is what the definition looks like. Why the extra colons? Because parsing `f<T>(args)` in an expression position would require unbounded lookahead to determine the meaning of the left angle bracket -- is it the beginning of generics or less-than? Therefore, even though Rust could be modified to accept`f<T>(args)` as a valid syntax when calling the function, the language team decided to require the colons in order to improve worst case parser performance.